Quantum noise in multipixel image processing

We consider the general problem of the quantum noise in a multipixel measurement of an optical image. We first give a precise criterion in order to characterize intrinsic single-mode and multimode light. Then, using a transverse mode decomposition, for each type of possible linear combination of the pixels' outputs we give the exact expression of the detection mode, i.e., the mode carrying the noise. We give also the only way to reduce the noise in one or several simultaneous measurements.